Commonly, power amplifiers (PA) suffer from low power efficiency at medium output power, because their load-line is typically optimized for maximum output power. Concepts of load-line switching and load-line adaptation are known. For instance, PIN-diode switched capacitors have been implemented in a BGY270 power amplifier of a transmitter for use in the GSM (Global System for Mobile communications)-system.
European patent publication 1 401 047 and U.S. Pat. No. 6,977,562 relate to “self-actuation” of RF-MEMS, wherein a variable capacitor is used in combination with a fixed matching network to set a load-line of a power amplifier adaptively. The fixed network is to be implemented with “a plurality of transmission lines,” without indicating any special property of the network.
In principle, an adaptation of the load-line as a function of output power can be applied to improve power amplifier efficiency at medium power levels, as may be gathered from G. Leuzzi, C. Micheli, “Variable-load constant efficiency power amplifier for mobile communications applications,” 33rd European Microwave Conference, pp. 375-377, Munich, 2003. According to theory and simulations, a single L-network gives excessive insertion loss for large impedance transformation ratios.
Moreover, network analysis reveals that the implementation of a variable inductor as a combination of a fixed inductor and variable capacitor makes insertion losses even worse. This is very undesirable, because it results in a huge PA efficiency degradation at high output power, for which a large impedance transformation ratio is typically required.
Most power amplifiers suffer from low efficiency at medium output power levels, because their load-line is optimized for maximum output power. Also most power amplifiers are often operated at medium power levels and only occasionally at maximum power. Further, for a given quality factor Q of the circuit elements, network insertion losses increase for increasing impedance transformation ratio. Consequently, losses of a variable matching network tend to become largest at maximum output power. In addition, power amplifier matching networks have to provide large rejection of harmonics, commonly realized with LC-resonance circuits tuned for these harmonic frequencies. Hence, realization of an output matching network with fixed LC-resonance circuits is in contradiction to the need of a variable LC-network for tuning the load-line at the fundamental frequency.